Thrombosis is a major cause of human disease. The interactions between human polymorphonuclear leukocytes (PMN) and platelets in the thrombotic process are not understood. We have found that the PMN proteolytic enzyme elastase is released during blood clotting, and that the purified enzyme specifically inhibits thrombin induced platelet aggregation. Therefore, we propose to test the hypothesis that elastase modulates platelet function by altering platelet surface glycoprotein structure. To investigate these structural changes, whole platelets will be radiolabelled and treated with elastase prior to SDS-PAGE, radioautography, and laser densitometry. To explore the significance of the structural modifications, the major glycoproteins enzymatically removed from the platelet surface by elastase will be purified by FPLC and used to prepare monospecific and monoclonal antibodies. These antibodies will be used to probe platelet function and to identify by Western Blot technique the parent surface glycoproteins of origin. To investigate the effect of elastase on purified membrane proteins, platelet glycoproteins Ib, IIb, IIIa, and V will be purified and treated with elastase. The results will be compared to elastase digests of labelled whole platelets. The effect of elastase on platelet function will be investigated by quantifying the binding of 125I-thrombin to platelets after treatment with elastase. We propose to investigate the inhibition of binding of 125I-thrombin by (1) purified preparations of the glycopeptides enzymatically removed by elastase and (2) antibodies against these glycopeptides. The effect of elastase on thrombin-induced platelet aggregation and secretion will be explored by similar techniques. These studies will be expanded to investigate the role of elastase in cell-cell interactions between PMNs and platelets. The research presented in this proposal may serve to establish possible new biochemical interrelationships between leukocytes and platelets.